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MENDEL AND THE GENE IDEA
Unit 3 - Chapter 14
Gregor Mendel
Augustinian, Teacher and Scientist
1822 - 1884
•Born in Heinzendorf Moravia in what is now the Czech
Republic.
•Received agricultural training
•Entered the Augustinian Monastery of St. Thomas at Br?nn
(now Brno) at the age of 22 and was ordained to the
priesthood in 1847.
•Taught science in secondary school fot the next 20 years
•Performed experiments in the monastery garden which were
to result in his discovery of the basic laws of heredity. His
findings were published in 1866. Using the garden peas for
his subject, Mendel's studies in "plant hybridization" proved
the existence of paired elementary units of heredity (now
called genes) and established the statistical laws governing
them.
•Subsequent scientists have refined his conclusions and
discovered the system of particulate heredity by units or
genes.
WHY PEA PLANTS?
• He already knew a lot about plants
• Pea plants come in many varieties, so
many traits can be studied
• Pea plants have “perfect” flowers – both
sex organs in the same flower. This gave
him complete control over the plant mating
STYLE
CARPEL
Characteristics
•Eye Color
•Eye shape
•Skin Color
•Skin type / texture
•Hair Color
•Hair type/ texture
•Shape of mouth
•Length of nose
•Nostril size
•Height of cheekbones
•Jawbone width
Traits
Blue-eyed
Widow’s Peak
Brown-eyed
Attached Lobe
Unattached Lobe
Homologous chromosomes, genes, alleles
Genetic Crosses
• The hybridization (mating) of
two traits of one
characteristics
For example: Blue-eyed man
mates a brown-eyed woman
OR
White-flowering pea plant
mated with a purpleflowering pea plant
Genetic Crosses, cont’d
• Monohybrid – only traits of one characteristic
are compared
• Dihybrid – traits of two characteristics are
compared
Genetic Terminology
1.
2.
3.
4.
5.
6.
7.
Dominant – an allele that expresses itself
instead of it homologue
Recessive – an allele that is silent, or whose
homologue is expressed
Homozygous – having two identical alleles of
the same gene
Heterozygous – having two different alleles of
the same gene
Genotype – the genetic (allelic) makeup of an
organism
Phenotype – the expression of the alleles (This
is a result of genotype)
True breeding – when the trait in question has
been observed for many generations (pure
bred)
The 7
Characteristics
Mendel Chose
Experimental Method
Some Basics Before We Move Along
The Generations
• P = Parental generation
• F1= First Filial or 1st generation of offspring
• F2 = Second Filial or 2nd generation of offspring
How does one determine what kind of
gametes will be formed?
2n
Number of heterozygous chromosome pairs
This is a constant (number of homologues)
This formula can only be applied to non-linked genes.
Independent Segregation of Alleles
The Punnett Square
Reginald C. Punnett,
inventor of the
Punnett Square
Mendel’s Monohybrid cross
Mendel’s
Dihybrid
cross
Mendel’s
Test Cross
When Mendel did not
know the genotype of an
individual expressing a
dominant trait, he did a
test cross by crossing the
individual with a
homozygous recessive
for the trait.
Recessively Inherited Diseases in Humans
• The disease occurs only if the individual inherits
2 recessive alleles from his parents
• If the individual has only one recessive allele, he
is not affected, but is considered a “carrier”
Examples: Tay Sach’s, Cystic Fibrosis, Sickle-cell
anemia
Dominantly Inherited Diseases in Humans
• The individual needs only one harmful allele to be
affected
• Lethal diseases inherited in this manner are less
common because its effects are obvious (except for
Huntington’s Disease – nervous system degeneration –
due to its late onset in life at ~age 45)
• Examples of Non-lethal diseases: Achondroplasia
(dwarfism)
When are Punnett Squares not useful?
When only two pairs of genes are involved, you can solve the problems
with Punnett squares.
With three or four or more pairs of genes, the phenotype distribution
begins to resemble a bell-shaped curve; then it's easier to estimate or
"eye-ball" predictions.
Other Patterns of Inheritance
•
•
•
•
•
•
Incomplete dominance
Co-dominance
Pleiotropy
Epistasis
Polygenic Inheritance
Sex-Linked Inheritance (X-linked)
Discussed in next chapter
Incomplete Dominance
This is when one allele is “silent” and the other allele cannot
produce the complete effect on its own.
Co-Dominance (Multiple Alleles)
Pleiotropy
When one gene causes multiple effects: Sickle Cell Anemia
Epistasis
When one gene at one
locus alters the
Phenotypic expression of
another gene at a different
locus
POLYGENIC INHERITANCE
This is when the phenotype is affected by more than one pair of
genes. This affects other characteristics like skin color, shoe size
and intelligence
Today, scientists
have actually
located 3 genes
that play a role in
eye color
determination.
GENETIC TESTING AND COUNSELING
• Carrier Recognition
• Fetal Testing
- Amniocentesis
- Chorionic Villus Sampling (CVS)
• Newborn Screening
THE END